Method, system, and program for error recovery while decoding compressed data

ABSTRACT

Provided are a method, system, and program for decoding compressed data. Compressed data is received and decoded. An error is detected while decoding a first location in the compressed data. A reentry data set is accessed having a pointer to a second location in the compressed data following the first location and decoding information that enables decoding to start from the second location. The second location in the compressed data is accessed and the decoding information in the accessed reentry data set is used to continue decoding the compressed data from the second location.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of the commonlyassigned patent application entitled Method, System, and Program forDecoding Compressed Data at Multiple Points, having U.S. applicationSer. No. 09/697,544, filed on Oct. 26, 2000 now U.S. Pat. No. 6,690,832.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a method, system and program for errorrecovery while decoding compressed data.

2. Description of the Related Art

Digital images may use one or more bits to describe the color intensityat each pixel. The term “pixel” as used herein refers to one or moreintensity inputs or bit values at a data point that represents data tobe rendered (i.e., printed, displayed, etc.), where the data to berendered may include, but is not limited to, images, text, compositeimages, graphs, collages, scientific data, video, etc. A pel is apicture element point that may be expressed with one bit. If only onebit is used to express the intensity, then the image is a bilevel imagewhere there are two possible intensity values per pixel, such as blackand white or full saturation and no intensity. Digital monochrome imagesthat allow for more than two intensities per pixel express theintensities as shades of grey.

Most systems compress image data before transmitting the data to anoutput device, such as a printer or display, that renders the imagedata. The output device must decode or decompress the compressed imageto output to print or otherwise render. Compressed images may also bearchived and then at some later time transmitted to an output device fordecompression and rendering, e.g., printing or displaying. For instance,an Adaptive Bi-Level Image Compression (ABIC) algorithm of the prior artwould sequentially encode each bit of image data by using the sevennearest neighbor bits and a probability distribution that is calculatedbased on previously coded data. In current implementations, the ABICdecoder maintains a history group of bits comprising the last N+2decoded bits. In certain current implementations, the ABIC decoder usesseven of the bits, including the last two decoded bits and bits in thehistory range from the (N−2) bit to the (N+2) bit. These are the sevennearest bits in the raster image. Details of using the ABIC algorithm toencode and decode data are described in the IBM publication entitled “AMulti-Purpose VLSI Chip for Adaptive Data Compression of BilevelImages”, by R. B. Arps, T. K. Truong, D J. Lu, R. C. Pasco, and T. D.Friedman, IBM J. Res. Develop., Vol. 32, No. 6, pgs. 775-795 (November1988) and the commonly assigned U.S. Pat. No. 4,905,297, whichpublication and patent are incorporated herein by reference in theirentirety.

If an error is encountered, the data used by the decoder to decompressthe compressed data, including neighbor bits and a probabilitydistribution, may be corrupted. To recover from an error, the decodermust begin decoding from a beginning point, such as the beginning of thecurrent image being decompressed. Prior art encoding schemes also encoderesynchronization data into the data stream to allow for decoding tobegin at a resynchronization point. For instance, the compressionschemes for Group 3 facsimile machines, including Modified Huffman (G3MH) and Modified READ (G3 MR), which were finalized in the CCITT StudyGroup XIV in the late 1970s, encode end-of-line (EOL) codes into thedata to allow resynchronization from the one-dimensional EOL points.

The G3 MH scheme independently codes horizontal runs of black or whitepels alternated across a page. Every compressed line of the black/whitefacsimile image ended with an unique end-of-line (EOL) code consistingof at least 10 (or eleven) zeros followed by a one. No valid combinationof run codes generated more than nine (or ten) zeros in a row. This EOLcode allows for resynchronization after every compressed line. Thetwo-dimensional G3 MR algorithm encodes each line with an EOL codefollowed by a tag bit specifying whether the next line was coded in oneor two dimensions.

These early Group 3 digital facsimile machines had no error correction.The receiver could not request a retransmission. The receiver couldresynchronize and recover from errors at the next one-dimensionallycoded line. Because the standard size facsimile page had 1728 pels/line(i.e. 216 bytes/line) this synchronization occurred quite frequently.Further, there is no standardized technique for handling incorrectlines. Some machines print the bad data generating streaks across thepage.

Other machines skip the erroneous lines and output squished lines oftext. Still other 1 machines replicate the previous line in order tomaintain consistent character height.

The CCITT Group 4 digital facsimile machines developed in the 1980sutilized the Modified Modified READ (G4 MMR) data compression algorithm.Instead of periodically coding lines one-dimensionally, the G3two-dimensional coding scheme is used on every line without any EOLs.Since these machines were designed for use on the digital data networks,the transmission was expected to be error-free so error recoveryresynchronization codes are not encoded into the data duringcompression.

The Joint Photographic Experts Group (JPEG) international datacompression standard designed for continuous-tone (contone) picturesprovides for optional resynchronization codes that may be encoded intothe data. These resynchronization codes are defined as Restart Markers(RSTm 0xFFD0-0xFFD7) and can be used to separate independently codedblocks of data. The Define Restart Interval (DRI 0xFFDD) markerspecifies how many blocks are coded between Restart Markers. If RestartMarkers are not encoded into the data, then decoding must restart at thebeginning of the JPEG image, from the Start of Scan marker.

Thus, with all the above techniques, resynchronization codes are encodedinto the actual compressed data to allow for error recovery whiledecoding at a point within the compressed data. Notwithstanding, thereis a continued need in the art for improved techniques for allowing forerror recovery during digital data transmissions.

SUMMARY OF INVENTION

Provided are a method, system, and program for decoding compressed data.Compressed data is received and decoded. An error is detected whiledecoding a first location in the compressed data. A reentry data set isaccessed having a pointer to a second location in the compressed datafollowing the first location and decoding information that enablesdecoding to start from the second location. The second location in thecompressed data is accessed and the decoding information in the accessedreentry data set to continue decoding the compressed data from thesecond location.

In further implementations, the compressed data is transmitted over anetwork from a transmitting system. In such case, a request may be sentto the transmitting system for a retransmission of compressed dataincluding the first location after detecting the error. A block of thecompressed data starting at a third location in the compressed data isreceived, wherein the pointer in one reentry data set addresses thethird location, and wherein the block of the compressed data includesthe first location. The decoding information in the reentry data sethaving the pointer to the third location is used to decode the block ofthe compressed data including the first location.

Further provided are a method, system, and program for caching data.Compressed data and reentry data sets are loaded into cache from anon-volatile storage device, wherein each reentry data set has a pointerto one location in the compressed data and decoding information thatenables decoding to start from that location. A request for decoded datais received, such that the cached data includes the compressed requesteddata. The uncompressed data is returned from the cache by decoding onlyparts of the cache, accessed via the reentry data sets, that correspondto the requested data.

In further implementations, if the requested data is not in uncompressedformat in the cache, then a determination is made of a first location inthe compressed data whose decoded output comprises the requested data. Adetermination is made of a reentry data set whose pointer addresses asecond location in the compressed data preceding the first location. Thedecoding information in the determined reentry data set is used todecode the compressed data from the second location through the firstlocation to output the requested data in uncompressed format. Therequested data is returned in the uncompressed format.

Yet further provided are a method, system, and program for transmittingdata in a compressed format. Compressed data is transmitted to areceiving device. A reentry data set is also transmitted to thereceiving device. The reentry data set has a pointer to a location inthe compressed data and decoding information that enables decoding tostart from the second location, wherein the receiving device is capableof using the decoding information in the reentry data set to decode thecompressed data from the location addressed by the pointer in thereentry data set.

In certain implementations, in the event of a transmission error, thereceiving device can resume decoding with the next reentry point afterthe error in the data. If the receiving device can ask forretransmission of the corrupted data, only the data in the corruptedreentry segment(s) need be retransmitted.

The described implementations provide a technique for decodingcompressed data using reentry data sets to allow decoding to begin fromone or more locations within the compressed data without having to startdecoding from the beginning of the compressed data. Theseimplementations may be used for data recovery to skip a location in thecompressed data having corrupt data.

BRIEF DESCRIPTION OF DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIGS. 1A and 1B illustrate computing environments in which aspects ofthe invention are implemented;

FIG. 2 illustrates an arrangement of bit planes in a manner known in theprior art;

FIGS. 3A and 3B illustrate the history bit values maintained for use indecoding compressed data in a manner known in the prior art;

FIGS. 4A and 4B illustrate different implementations of logic togenerate reentry data sets in accordance with preferred embodiments ofthe present invention;

FIGS. 5A and 5B illustrate different implementations of logic to use thereentry data sets to decompress and output multi-bit pixels inaccordance with preferred embodiments of the present invention;

FIG. 6A illustrates a computing environment in which aspects of theinvention are implemented;

FIG. 6B illustrates a format of a reentry data set data structure inaccordance with implementations of the invention;

FIG. 7 illustrates logic to decode compressed data and recover fromerrors in accordance with certain implementations of the invention;

FIG. 8 illustrates logic to decode compressed data and recover fromerrors in accordance with still further implementations of theinvention;

FIG. 9 illustrates a computing environment in which further aspects ofthe invention are implemented;

FIG. 10 illustrates a computing environment in which still furtheraspects of the invention are implemented; and

FIG. 11 illustrates logic to decode compressed data within the computingenvironment illustrates in FIG. 10.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings which form a part hereof and which illustrate severalembodiments of the present invention. It is understood that otherembodiments may be utilized and structural and operational changes maybe made without departing from the scope of the present invention.

FIG. 1A illustrates a computing environment in which preferredembodiments are implemented. A reentry decoder 100 receives a compresseddata stream 102, which in preferred embodiments comprises a concatenateddata stream, such as shown in FIG. 2 above, compressed using a standardABIC algorithm known in the art. The reentry decoder 100 decompressesthe compressed data stream 102 in the manner described below to generatereentry data sets 104 to allow a subsequent decoding operation toindependently decode from different points in the compressed data stream102. A reentry data set is provided for each plane 4, 6, 8 in thecompressed concatenated bitmap. In the case that there are only twoconcatenated planes, only one reentry data set is needed to decode atthe first bit in the second plane. As discussed, a reentry data set isnot needed to decode the first plane because current decoding algorithmsassume a default value of history bits to decode the first line of bitsin the first plane. Likewise more planes require more reentry data sets.

The reentry data set for a plane comprises an offset or pointer to aposition in the compressed data whose decompressed output comprises onebit value for the first bit in one of the planes 4, 6, and 8; a historyline of decoded bits, and 128 probability estimates. The history linecomprises a last line of N bits plus two decoded bits, wherein N is thenumber of bits per line in the bit plane. The seven nearest neighborbits used in the decoding comprise the following bits from the historyline, the last two decoded bits and the bit range from the (N−2) bit tothe (N+2) bit. FIG. 3A illustrates the seven history bits used to decodebit “X” in the event that the bit to decode is in the middle of a lineof the bit plane, which would comprise the two most recent decoded bitsi and j, as well as the range of bits from the previous line, whichcomprise bits a through e. FIG. 3B illustrates a line of the last N plustwo bits, where N is eight. The bits in FIG. 3B used to decode “X” wouldcomprise the last two decoded bits i and j and the last sixth to tenthbit, or bits a through e. In this way, the nearest bits are defined inraster scan order. The bit selection of bits to use to decode one bitshown in FIGS. 3A and 3B are used in the current art to select bits fromthe last N plus two history bits to use in the ABIC decoding operation.The seven selected neighbor bits are used to select one of the 128probability estimates to use to decode the first bit in plane 4 in amanner known in the art.

When coding the concatenated data stream, a coder (not shown) usesarithmetic coding to code a particular bit value based on seven neighborbits and a probability estimate selected based on the value of the sevenneighbor bits in a manner known in the art. The decoder that decodes theconcatenated ABIC data stream looks at the seven neighbor bits anddetermines a probability estimate for the current bit being considered.The decoder then uses the probability estimate to decode the actual bitvalue. In this way, in a manner known in the art, the coding anddecoding systems use the same statistical model or dynamic probabilityestimation process to determine how each coding decision is“conditioned” on prior coding decisions. Thus, the reentry decoder 100would use the same dynamic probability estimation process to determinethe probability estimate based on past data used by the coder whencoding/compressing the data stream 102 in a manner known in the art.Further details of decoding and encoding the data stream are describedin U.S. Pat. No. 4,905,297 and the IBM publication A Multi-Purpose VLSIChip for Adaptive Data Compression of Bi-Level Images, both incorporatedby reference above. Further, JPEG: Still Image Data CompressionStandard, by William B. Pennebaker and Joan L. Mitchell (Van NostrandReinhold, 1993), at pages 409-430 describe arithmetic binary decodingand encoding.

In preferred embodiments, the history data for the first bit in thefirst plane 2 is assumed to be all zero data. After generating reentrydata sets 104 for the first bit on the second plane 4, third plane 6,and fourth plane 8, the reentry decoder 100 transfers the receivedcompressed data stream 102 and calculated reentry data sets 104 to anoutput device 110, which may comprise any device known in the artcapable of rendering input data, such as a printer, display, a storagedevice for future rendering, etc. The entire compressed data stream 102and reentry data sets 104 are stored in buffer 112 of the output device110. The output device 110 includes a decoder 114 that accesses thereentry data sets 104 in the buffer 112 to decompress one or more linesof bit data from each of the four planes in the compressed data stream102. The decoder 114 stores one or more lines of decompressed data fromeach plane in the respective plane buffer 116 a, b, c, d. Once thedecoded bit data for one or more lines from each plane 2, 4, 6, 8 istransferred to the buffers 116 a, b, c, d, respectively, the completefour bits for one or more pixels may then be transferred to the outputdevice and rendered or further processed, e.g., halftoned, screened,dithered, etc., before being rendered.

The decoder 114 may use multiplexing logic to sequentially decode one ormore lines of bit data in each plane. Alternatively, the decoder 114 maybe comprised of separate decoders to decode in parallel the bit data foreach plane. The buffers 112 and 116 a, b, c, d may be part of the samememory device or separate hardware buffers or memory devices. Thereentry decoder 100 and decoder 114 may comprise an ABIC decoder, or anyother decoder known in the art. The reentry decoder 100 may be locatedeither external or internal to the output device 110. If the reentrydecoder 100 is located internal to the output device 110, then thereentry decoder 100 unit may be separate from the decoder logic 114 orboth may be part of the same logic unit. In alternative embodiments, theencoder could save-off the reentry data sets when coding the data streamand provide the reentry data sets to the decoder 114 to use to decodethe compressed data stream at multiple points. This alternativeimplementation avoids the need for a reentry decoder 100 as the encoderis used to generate the reentry data sets.

FIG. 1B. illustrates an alternative computing environment in whichfurther aspects of the invention are implemented. A reentry decoder 130receives compressed data 132, which in alternative preferred embodimentcomprises Huffman-encoded baseline JPEG data. The reentry decoder 130partially decompresses the compressed data stream 132 in the mannerdescribed below to generate the reentry data sets 134 to allow asubsequent decoding operation to independently decode from differentpoints in the compressed data stream 132.

A reentry data set 134 in the implementation of FIG. 1B is composed ofthe bit pointer to the compressed data at the Minimum Coded Unit (MCU)boundary, the DC predictor values for all the components in the MCU andthe output location of the MCU. A Forward Discrete Cosine Transform(FDCT) creates 64 coefficients. The first coefficient in the upper righthand corner is for zero frequency and comprises the DC coefficient orpredictor. The other of the 64 coefficients are called AC coefficients.Each reentry segment contains an arbitrary number of MCUs. The reentrydecoder 130 needs only to partially strip the Huffman entropy coding.After generating reentry data set 134, the reentry decoder 130 transfersthe compressed data stream 132 and the reentry data sets 134 to anoutput device 140, which may comprise any device known in the artcapable of rendering input data, such as printer, display, a storagedevice for future rendering, etc. The entire compressed data stream 132and the reentry data sets 134 are stored in the buffer 142 of the outputdevice 140. The output device 140 includes a decoder 144 that decodesthe compressed data stream 132. On encountering an error, the decoder144 recovers by advancing to the next reentry point specified in thereentry data sets 134, uses the information to reestablish thepredictors and the start of the MCU block, and continues decoding.

FIG. 4A illustrates logic implemented in the reentry decoder 100 todecode the compressed data 102 for the purpose of generating reentrydata sets 104 for the decoder 114. Control begins at block 200 with thereentry decoder 100 receiving the compressed data 102. The reentrydecoder 100 decodes (at block 202) a plane of bits using a line bufferfor the history data in a manner known in the art. The reentry decoder100 uses a rolling buffer and overwrites previously decoded data as eachbit is decoded to maintain the last N plus two bits, where N is thenumber of bits in a line of the planes. To determine when a plane ofbits has been decompressed, the reentry decoder 100 would use the knownheight and width dimensions of the planes in the concatenated compresseddata stream 102, and the number of lines in each plane. By keeping trackof the cumulative number of decompressed lines, the reentry decoder 100could determine whether a decompressed line is the last line in a plane.Alternatively, there may be an end of plane marker in the compresseddata stream 102, which the reentry decoder 100 would use to determinewhen the last line of a plane has been decompressed.

After decoding the last bit on the plane and buffering the last N plustwo decoded bits including the last decoded bit on the plane, thereentry decoder 100 generates (at block 204) a reentry data setcomprising a pointer to a location in the compressed data stream whosedecoded output is the first bit in one plane 4, 6, 8 following the firstplane 2, the last N+2 decoded bits preceding the first bit in a plane,the 128 probability estimates, and any register values used duringdecoding. For instance, the Q-coder used in ABIC coding/decodingmaintains A and C registers indicating an interval of bits being coded.If (at block 206) the next plane is the last plane to decode, then thereentry decoder 100 transfers (at block 208) the compressed data 102including any generated reentry data sets 104 to the buffer 112 in theoutput device 110 and ends. As discussed, the first plane 2 in thecompressed data stream 102 is assumed to have initialized values priorto the first bit in the first plane, such that the last N plus two bitsin the history line for the first plane 2 all have zero value.

With the logic of FIG. 4A, the reentry decoder 100 does not need a largebuffer because it only needs to buffer the last N plus two decoded bitsat a time for each bit decoded in addition to the statistical data andother information needed to continue decoding, such as probabilityestimates, register values, etc.

After generating the reentry data sets 104, the reentry decoder 100transmits the compressed data 102 and reentry data sets 104 to thedecoder 114. In preferred embodiments, the reentry decoder 100 transfersthe data stream in compressed format regardless of whether the reentrydecoder 100 is located internal or external to the output device 110 tominimize the transfer time to the buffer 112 used by the decoder 114.

FIG. 4B. illustrates the logic to generate the reentry data sets if thedata is compressed using a baseline JPEG algorithm. Control begins atblock 220 with the reentry decoder 130 receiving the compressed data132. The reentry decoder 130 partially decodes the current MCU in themanner known in the art at the block 222. The DC terms of each block aredecoded in full for every block. The AC components need not be fullydecoded. An RS byte must be parsed, but the extra bits that follow areskipped. To encode the DCT block efficiently using the Huffman entropycoding, the AC coefficients are expressed as a set of RS byte+extra bitsterms. The RS byte has the R nibble and S nibble. “R” means “Run” andindicates how many zero AC terms precede the current term, up to 15. Iftwo nonzero AC terms follow one another, the second will have the R ofzero. The “S” nibble gives the size of the AC term described by theentry. If S is zero, then the current AC term is also zero, R is 15 andthe run is 16 terms long. A nonzero value for S indicates the number ofbits following the RS entry. These bits can be used to decode thecurrent AC coefficient. For partial decoding, the RS byte is decoded andparsed. The R position indicates how many coefficients are skipped andthe S position indicates the number of following bits. An RS of 0x00indicates the end of a block (i.e., all the remaining AC coefficientsare zero).

The reentry decoder 130 parses the RS byte to locate the end of theblock and thus the DC term of the next block. In block 224 the reentrydata set is generated comprising of the bit pointer to the start of theMCU block, the DC predictors for all of the components in the MCU andthe output position of the MCU block. The compressed data for thecurrent MCU is then transferred (at block 226) to the output device 140,together with the reentry data set 134. At blocks 228 and 230, anyfurther compressed MCUs are also decoded. In certain implementations,the reentry data set 134 may not be transferred with the every MCU inthe data stream, but only at selected points in the data. If there aremore MCUs in the image, control proceeds back to block 222 to decode thenext MCU.

FIG. 5A illustrates logic implemented in the decoder 114 to decompressthe compressed data stream 102 using the generated reentry data sets104. In FIG. 5A, the decoder 114 multiplexes between decoding L lines ofbits in each plane, where L comprises one or more lines. Control beginsat block 250 with the decoder 114 receiving and buffering in buffer 112the compressed data stream 102 and reentry data sets 104. The decoder114 performs a loop at blocks 252 to block 264 to multiplex the decodingof L lines in each of the four planes. If (at block 254) the decoder 114is starting the decoding process at the first bit in the first plane,then the decoder 114 decodes/decompresses (at block 256) the first Llines of bits in the first plane in a manner known in the art todecompress the initial bits in a compressed data stream. As discussedwhen decompressing the first line in the data stream 102, the decoder114 would assume that the previous line included all zero bit values anduse initial values for register values and the probability estimateswhen decoding a first bit of an ABIC compressed data stream.

If (at block 254) the decoder 114 is not decoding the first line of thecompressed data 102, then the decoder 114 uses (at block 258) thepointer in the reentry data set for plane i to access the compresseddata stream 102 at the location addressed by the pointer. The decoder114 then uses (at block 260) the N plus two history bits, theprobability estimates, and the register values saved with the reentrydata set for plane i to decode the next L lines of bits in plane i in amanner known in the art. The decoded L lines of bits are stored in theircorresponding plane buffer 116 a, b, c, or d. After decoding the Llines, the decoder 114 updates (at block 262) the reentry data set forplane i to include decoding information at the state of the last bitdecoded in plane i, including a new pointer to a location in thecompressed data stream whose output is the bit following the lastdecoded bit, the previously decoded N+2 bits, the 128 probabilityestimates, and current register values. In preferred embodiments, areentry data set for the first plane is first populated with data afterthe first L lines of bits in the first plane have been decoded usingdefault initialization values. In this way, the decoder 114 can shift toprocessing bits in the next plane (i+1) and use the updated reentry dataset information to later proceed directly to decode the next L lines ofbits in plane i after completing the decoding of L lines in the otherplanes. At block 264, the decoder 114 proceeds back to block 252 tomultiplex through the next L lines from the next plane.

After decoding the same L lines from each of the four planes, thedecoder 114 reassembles (at block 266) the buffered bits in the fourplanes so each pixel value has four-bit values, one bit from each plane.The reassembled bits for the pixels in the L lines are then outputted(at block 268), where they may be rendered or further processed, such asscreened, halftoned, dithered, etc. If (at block 270) there are furtherlines of bits in each plane to decompress, then control transfers toblock 252 to decode/decompress the next L lines of bits in each plane.In further embodiments, the decoder 114 may begin further decodingoperations during the process of reassembling and outputting thebuffered bit values.

In the logic of FIG. 5A, the decoder 114 multiplexed decoding operationsby sequentially decoding L lines from each plane, outputting the decodeddata, and then proceeding to decode the next L lines in each plane. Inalternative embodiments, the decoder 114 may comprise four separatedecoding units to allow for the parallel decoding and buffering of Llines of bits at a time in each plane.

With the logic of FIG. 5A, the decoder 114 outputs decompressed datafaster than techniques known in the art because the decoder 114 canoutput L lines of data from each plane before having to complete thedecoding of the first three planes. In the prior art, the decoder mustbuffer the decoded first three planes of data and lines from the fourthplane before data can be outputted. With the preferred embodiments, asmaller buffer size may be used to buffer decoded bits because thedecoder 114 only needs to buffer L lines of bits from each plane at atime, not the entire first three planes.

The preferred embodiments utilize the previously generated reentry datasets to allow the decoder to break into different parts of thecompressed data stream to decode and output L lines of bits from eachplane before proceeding with further lines in the planes. Forillustrative purposed a monochrome image with 4-bits per pixel has beenused. The preferred embodiments also apply to monochrome pixels withmore or less bits per pixel and to the bit planes created from themultiple components of a color image.

FIG. 5B illustrates the logic implemented in the decoder 144 if the datais compressed using a baseline JPEG algorithm. For the decompression,the compressed datastream can be treated as a sequence of compressedMCUs. To decode an arbitrary rectangular area of the image, the decoder144 must decode one or more disjoint subsequences of MCUs, eachrepresenting a set of output scanlines. The region need not be alignedon MCU boundaries. In that case, after decompressing each MCU, parts ofthe border MCUs that fall outside of the desired region are discarded.The algorithm illustrated in FIG. 5B also works correctly for thetrivial case, where the whole image is being decompressed.

The decoder 144 decodes K disjoint MCU sequences, with M MCUs in each.The control starts at block 274 with the output device 140 receiving andbuffering in buffer 142 the compressed data stream 132 and reentry datasets 134. The decoder 144 performs a loop at block 276 to 298 to decodeall the MCU sequences. Each loop iteration decodes a complete set ofscanlines in the output region. The decoder 144 first finds (at block278) the closest reentry point prior to the first block in the currentsequence. Note that the image beginning and start of any JPEG restartintervals are by definition “restart pointers”. The MCU referenced bythe reentry point becomes the current MCU. If, at block 282, the currentMCU is not the first MCU of the sequence to be decoded, then the currentMCU is partially decoded as described in FIG. 4B and skipped (at block280). The next MCU becomes the current MCU and control returns to block282.

If (at block 282) the current MCU is the first MCU in the currentsequence of MCUs to be decoded, then the algorithm enters a loop atblocks 284 to 294 over MCUs in the sequence. The current MCU is decoded(at block 286). If (at block 288) the current MCU is a boundary MCU,then any decoded pixels out of the desired region are discarded (atblock 290). From blocks 288 or 290, the decompressed pixels are added(at block 292) to the output buffer. At block 294, control proceeds backto block 284 if there are any further MCUs to process, such that thenext MCU becomes the current MCU. Otherwise after all MCUs in thesequence are processed, the current set of scan lines is outputted (atblock 296) and control returns (at block 298) to block 276 to processthe next MCU sequence. After processing all MCU sequences, control ends.

Using the algorithm of FIG. 5B, not all the image must be decoded togenerate a rectangular region. Some extra MCU blocks may have to bepartially decoded, depending on the position of the region relative tothe reentry points. The extra blocks are decoded just enough that theycan be skipped, in the manner described in FIG. 4B.

Using Reentry Data Sets for Error Recovery and Other Purposes

In the above described implementations, reentry data sets were used toallow for simultaneous decoding of data in four different data planesand decompressing an image area without decompressing the whole image.Following are some additional uses of reentry data sets.

FIG. 6A illustrates one computing environment including a transmittersystem 300 that transmits compressed data 302 and data reentry data sets304 a, b . . . n. With respect to FIG. 6B, each reentry data set 304 a,b . . . n comprises a pointer 350 or offset (shown as pointers 350 a, b. . . n in FIG. 6A) to a position in the compressed data whosedecompressed output comprises one bit value in the data stream, ahistory line of decoded bits 352, probability estimates 354, andregisters 356. More, less or different types of data may be included inthe reentry data sets 304 a, b . . . n, depending on the compressionscheme used for the compressed data 302. For instance, in an ABICdecoding operation, 128 probability estimates are provided and thehistory line comprises a last line of N bits plus two decoded bits,wherein N is the number of bits per line in the bit plane. The sevennearest neighbor bits used in the decoding comprise the following bitsfrom the history line, the last two decoded bits and the bit range fromthe (N−2) bit to the (N+2) bit. Alternative compression schemes mayrequire different types and/or additional history and statistical datato begin decoding from a specific location within the compressed data502.

The transmitter system 300 may comprise any type of computer orelectronic device that is capable of transmitting digital data over anetwork 306. The network 306 may comprise any type of data communicationnetwork, including a wireless network (e.g., telephone communicationsystem, cellular communication system, digital radio, digitaltelevision, satellite, infrared, etc.) or a wired network (e.g., theInternet, an Intranet, Local Area Network (LAN), storage area network(SAN)).

The transmitter system 300 includes the capability to transmit data overthe network 306 and a receiving system 308 includes the capability toreceive data transmitted, such as compressed data 302 and reentry datasets 304 a, b . . . n, over the network 306. In certain implementations,the receiving system 308 is capable of bidirectional communication withthe transmitter system 300 and in other implementations, the receivingsystem 308 can only receive data from the transmitter system 300, suchas the case with wireless data broadcasts over a radio, satellite orother broadcasting network.

The receiving system 308 may comprise any computing device known in theart, e.g., a computer, server, desktop system, telephony device, handheld computer, palm top, etc., capable of receiving data from thenetwork 306. The receiving system 308 further includes a decoder 310capable of decoding a stream of compressed data 302 and, if necessary,using the reentry data sets 304 a, b . . . n to access the compresseddata stream at the location addressed by the pointer in the reentry dataset 304 a, b . . . n to produce output data 312. The output data 312 maycomprise any combination of text, images, video, audio or any otherdigital output. The decoder 310 may be implemented as software code thatis executed by a processor (not shown) within the receiving system 310or as hardware logic, e.g., an Application Specific Integrated Circuit(ASIC), etc.

FIG. 7 illustrates logic implemented in the decoder 310 to decode thecompressed data 302 using the reentry data sets 304 a, b . . . nreceived from the transmitter system 300. At block 400, the decoder 310would begin decoding the compressed data 302 in a manner known in theart. Upon detecting (at block 402) an error while decoding thecompressed data, the decoder 310 determines (at block 404) an offsetinto the compressed data stream where the error occurred. The decoder310 then identifies (at block 406) the reentry data set having a pointer350 a, b . . . n that is the closest in all the reentry data sets 304 a,b . . . n to the location of the error and that follows the determinedoffset. The pointer in the determined reentry data set 304 a, b . . . nis then used (at block 408) to access the compressed data 403 at thelocation addressed by the pointer 350 a, b . . . n and the history data352, probability estimates and any other data in the determined reentrydata set 304 a, b . . . n is then used (at block 410) to begin decodingthe compressed data 302 from the location addressed by the pointer 350a, b . . . n. The decoder 310 then determines (at block 412) thelocation in the output data of the last decoded bit preceding the firstbit decoded using the determined reentry data set. A message is thensent (at block 414) to the transmitting system 300 for error recoveryincluding the identifier of the reentry data set 304 a, b . . . n dataset 304 a, b . . . n whose pointer 350 a, b . . . n is the closestpointer that follows the location in compressed data where erroroccurred.

Upon receiving the error message, in one implementation, the transmittersystem 300 would return a block of data between the pointers in thereentry data sets that are the closest preceding and closest followingthe error location in the compressed data stream. The transmitter system300 may also transmit the reentry data for the block of transmittedcompressed data to allow decoding from the beginning of theretransmitted block including the compressed data where the erroroccurred.

With respect to FIG. 7, at block 416, the receiving system 308 receivesthe retransmitted block of compressed data along with the reentry dataset for the data block. The decoder 310 would then use (at block 418)the history data and probability estimates in the received reentry dataset to decode the compressed data block. The decoded block is theninserted (at block 420) into the output data 312 at the location in thedecompressed data comprising the decoded compressed data 302 addressedby the pointer 304 a, b . . . n. When the error occurs, the decoderknows the offset into the both compressed and decompressed data, so themapping is immediately available.

In the implementation described with respect to FIG. 7, the decodedoutput is any data that is capable of being corrected. For instance, thedecoded output may comprise image or text data that can be updated.Further, the data can comprise any other type of digital data that isbeing buffered before being rendered, such as audio, video, image, text,etc. In such case, the recovered data decoded from the retransmittedblock of compressed data would be inserted into the buffer holding thedata to be rendered.

In alternative implementations, the decoder 310 may not attempt torecover from a data error. In such case, the reentry data sets 304 a, b. . . n would be used to restart decoding from the closest pointerfollowing the location of the error in the compressed data. This willallow the decoder 310 to continue decoding the compressed data followingthe location of the error without having to wait to receive aretransmission of data and then start decoding from the beginning of thecompressed data. Instead, with the described implementations, thedecoder 310 can skip the data with the error and proceed to the locationin the compressed data addressed by the pointer in the next reentry dataset to continue decoding the data. With this implementation, the outputdata will only miss that output from the point in the point in thecompressed data where the error occurred to the location addressed bythe next reentry data set 304 a, b . . . n.

FIG. 8 illustrates an alternative implementation of the logic in thedecoder 310. Initially, the transmitter system 300 transmits (at block450) the compressed data 302 without the reentry data sets. Uponreceiving (at block 460) the transmitted compressed data 302, thedecoder 310 continuously decodes (at block 460) the compressed data 300using decoding logic known in the art until an error is detected (atblock 462) in the compressed data 302. In such case, the decoder 310determines (at block 464) the offset location into the compressed data302 where the error occurred and sends (at block 466) a message to thetransmitting system 300 including the determined offset. At block 470,the transmitter system 300 receives the message from the receivingsystem 308 (sent at block 466) and determines (at block 472) the blockof compressed data between two reentry data set pointers 350 a, b . . .n that include the location of where the error occurred. The determinedblock of compressed data and the reentry data set 304 a, b . . . nhaving the pointer 350 a, b . . . n addressing the beginning of thedetermined block are sent (at block 474) to the receiving system 308.

At block 480, the receiving system 308 receives the resubmitted block ofcompressed data and reentry data set (sent at block 474) and proceeds toblock 418 in FIG. 7 to begin decoding data from the retransmitted blockof compressed data. After retransmitting the block of compressed data atblock 474, the transmitter system 300 begins (at block 482) transmittingcompressed data with the reentry data sets 304 a, b . . . n because theerrors in the previous transmission indicates an increased likelihood offuture transmission errors. Upon receiving the compressed data andreentry data sets (at block 490), the receiving system 308 proceeds toblock 410 in FIG. 7 to process the compressed data according to thelogic of FIG. 7. In such case, upon detecting future errors, thereceiving system 308 can continue decoding the data and producing outputdata using the reentry data sets to skip the block of data including theerror. The recovery of the corrupted data can be done prior to orconcurrently with decompressing the rest of the image in a manner knownin the art.

FIG. 9 illustrates an additional implementation including a storagemedium 500, such as a removable storage medium (e.g., tape cassette,optical disk, swappable disk, etc.) or non-removable storage medium(e.g., one or more hard disk drives, etc.) including the compressed data502. The reentry data sets 504 for the compressed data 502 may be storedin the storage medium 500 or an alternative location. A computer 506accesses data in the storage medium 500, including the compressed data502, via a storage device 508, such as a tape drive, optical disk drive,disk drive interface, etc. The computer 506 includes a decoder 510, suchas the decoder 310 described with respect to FIG. 6A, to decodecompressed data 502 to produce output 512 and use the reentry data sets504 to independently break-into a location in the compressed data 502 tostart decoding in the event of an error as described above. Inalternative implementations, the decoder 510 may be implemented in thestorage device 508, such that compression and decompression is handledby the storage device 508.

In certain implementations of FIG. 9, the storage medium 500 maycomprise a low-cost, high capacity storage medium, such as a tapecassette, optical disk, low cost hard disk drive, etc., to archive datafor extended periods of time, such as several years. One concern witharchiving data for extended periods of time is corruption of the storeddata resulting from physical defects that occur in the storage medium500 over time. The described implementations would allow access to thecompressed data 502 in the storage medium 500 even if certain data orsectors on the storage medium become corrupt. Upon reaching a locationin the storage medium 500 that is defective or where data is corrupted,the decoder 510 could use the reentry data sets 504 to skip thedefective/corrupted data and proceed to decode from a location in thecompressed data 502 addressed by a pointer in one reentry data setfollowing the defective location in the compressed data 502. Such animplementation would allow decoding of the compressed data 502 tocontinue beyond the location where the data is corrupt or storage medium500 is defective. In this way, any corruption of data over time does notrender all the archived data inaccessible.

The implementation of FIG. 10 may also apply to an environment where thestorage medium 500 comprises a hard disk drive that stores active datain a compressed format. For instance, the decoder 510 may execute in theoperating system kernel to decode compressed data 502 to return tooperating system or application program processes. The decoder 510 coulduse the reentry data sets 504 to provide the operating system andapplication processes continued access to data in the event that asection of the compressed data 502 is corrupt or stored on a defectivesector of the storage medium 500.

FIG. 10 illustrates a still further computing environment in which theinvention may be implemented. A computer system 600 would loadcompressed data 602 and reentry data sets 604 a, b . . . n for thecompressed data 602 from a storage device 606, such as a hard diskdrive, to a high speed cache 608, such as a volatile memory device. Thecomputer system 600 includes decoder logic 610 and a processor 612. Thedecoder logic 610 may comprise software code executed by the processor612 or any other component that performs cache 608 managementoperations. Alternatively, the decoder logic 610 may be implemented ashardware, such as an Application Specific Integrated Circuit (ASIC), orother integrated circuit type device. The cache 608 comprises higherspeed storage than the storage device 606. The decoder logic 610 iscapable of using the reentry data sets 604 a, b . . . n to decode fromdifferent locations in the compressed data 602 to produce uncompresseddata 610 in the cache 608.

In the implementation of FIG. 10, the processor 612 may load thecompressed data 602 into the cache 608 and decode in cache 608 using thereentry data sets 604 a, b . . . n. Storing compressed data 602 in thecache 608 increases the amount of data that may be stored in cache 608,thereby reducing the likelihood of cache misses (requested data is notin cache 608 and must be accessed from the slower storage device 606)and increasing the likelihood of cache hits (requested data can bereturned from the higher speed cache 608). Performance is enhanced byreducing cache misses and maximizing cache hits because data can bereturned to a requesting computer system 600 process faster from thehigher speed cache 608 than the slower speed storage device 606.

FIG. 11 illustrates logic implemented in the processor 612 and decoder610 to maintain compressed data 602 in the cache 608. At block 650, theprocessor 612 copies compressed data 602 and reentry data sets 604 a, b. . . n for the compressed data 602 into the cache 608. Upon receiving(at block 652) a request for data that is stored in compressed format602 in the cache 608, a determination is made of whether the requesteddata is uncompressed data 610 in cache 608. If so, the processor 610returns (at block 656) the requested uncompressed data 610 from thecache 608. Otherwise, if the requested data is not uncompressed in cache608, then the decoder 610 determines (at block 658) the location/offsetinto the compressed data 602 whose decoded output comprises therequested data. A determination is then made (at block 660) of thereentry data set 604 a, b . . . n having a pointer that is at or theclosest preceding pointer in the reentry data sets 604 a, b . . . n tothe determined offset. The decoder 610 then accesses (at block 662)compressed data at the location addressed by the pointer in thedetermined reentry data set 604 a, b . . . n and uses (at block 664) thehistory data and probability estimates therein to decode the compresseddata in cache 608 from the location addressed by the determined reentrydata set pointer. After decoding the requested data, control proceeds toblock 656 to return the uncompressed requested data from cache 608.

Additional Implementation Details

The decoding logic and operations described herein may be implemented asa method, apparatus or article of manufacture using standard programmingand/or engineering techniques to produce software, firmware, hardware,or any combination thereof. The term article of manufacture as usedherein refers to code or logic implemented in hardware logic (e.g., anintegrated circuit chip, Programmable Gate Array (PGA), ApplicationSpecific Integrated Circuit (ASIC), etc.) or a computer readable medium(e.g., magnetic storage medium (e.g., hard disk drives, floppy disks,tape, etc.), optical storage (CD-ROMs, optical disks, etc.), volatileand non-volatile memory devices (e.g., EEPROMs, ROMs, PROMs, RAMs,DRAMs, SRAMs, firmware, programmable logic, etc.). Code in the computerreadable medium is accessed and executed by a processor. The code inwhich preferred embodiments of the error recovery are implemented mayfurther be accessible through a transmission media or from a file serverover a network. In such cases, the article of manufacture in which thecode is implemented may comprise a transmission media, such as a networktransmission line, wireless transmission media, signals propagatingthrough space, radio waves, infrared signals, etc. Of course, thoseskilled in the art will recognize that many modifications may be made tothis configuration without departing from the scope of the presentinvention, and that the article of manufacture may comprise anyinformation bearing medium known in the art.

Preferred embodiments were described with respect to a printer outputdevice. However, the output values may be rendered using output devicesother than printers, such as such as display monitors, a storage devicefor future rendering, etc.

In preferred embodiment, the decoder and reentry decoder are implementedas hardware, e.g., a Field Programmable Gate Array (FPGA), ApplicationSpecific Integrated Circuit (ASIC), etc. In alternative embodiments, thedecoder and reentry decoder may be implemented as software executed by aprocessor.

Preferred embodiments were described with respect to a data stream ofconcatenated planes of bit values compressed using an ABIC algorithm.However, those skilled in the art will recognize that the preferredembodiment technique for generating and using reentry data sets may beused to decode concatenated planes coded using compression algorithmsother than ABIC, such as a Huffman coding, G4-MMR algorithm, etc. Inalternative algorithms, such as the G4-MMR algorithm, only the last lineof history bits are needed, as well as the pointer into the compresseddata stream of where to start decoding.

Preferred embodiments were described with respect to using the reentrydata sets to decode bit planes and also to decompress image areas indata streams compressed using a baseline JPEG algorithm. In furtherembodiments, the data stream being decompressed using the preferredembodiment reentry data sets may comprise of other types of imagestreams known in the art.

Preferred embodiments described information used by the decoder in thereentry data sets to decode from the location addressed by the pointeras N plus two previous bits, probability estimates and register values.However, the decoding information included with the reentry data setsmay comprise any information the decoder needs in order to begindecoding from the point in the compressed data stream addressed by thepointer.

In preferred embodiments, the image data was expressed as individualvalues for each bit in a bit plane, such as described with respect toFIGS. 1A, 1B and 2. In alternative embodiments, the bit values in eachplane may be expressed as transition points, such that each plane storesthe points at which the bit values transition from 0 to 1 or 1 to 0. Insuch embodiments, the decoder would decode the transition points in amanner known in the art, and then generate lines of bit values for thedecompressed transition points.

Preferred embodiments were described with respect to decompressing stillimage data comprised of bits. In alternative embodiments, the datastream subject to the decoding/decompression techniques of the preferredembodiments may comprise other types of data than still image data.

In preferred embodiments, the decoder would decode the same L number oflines from each plane before outputting the data. In furtherembodiments, the decoder may decode a different number of lines of datafrom each plane and then output reassembled lines of data for all planesfor the same lines of bits. Thus, the decoder may not output all thedecoded lines for each plane.

In preferred embodiments, the decoder would buffer lines of data fromeach plane, reassemble the lines from each plane, and then output thereassembled data from each plane. In alternative embodiments, thedecoder may output data from less than all the buffers.

In preferred embodiments, the decoder reassembled and outputted the samelines of bit data from each plane. In alternative embodiments, thedecoder may output from the plane buffers different lines of data orbits.

In preferred embodiments, the decoder decoded lines of data. Inalternative embodiments, the decoder may decode less than all the bitsin a line before proceeding to the next plane to decode bits.

Preferred embodiments were described with respect to four planes.However, in alternative embodiments, the concatenated data stream mayinclude more or less than four planes. Still further, the decodinginformation may provide more or less than N plus two bits of historydata. Yet further, the bit stream subject to decompression and reentrydoes not have to comprise concatenated planes.

The described implementations provided specific computing environmentsin which the decoder and reentry data sets may be used. Those skilled inthe art will appreciate that the decoder and reentry data sets describedherein may be used in various other computing environments where data isstored in a compressed format.

The foregoing description of the preferred embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto. The above specification, examples and dataprovide a complete description of the manufacture and use of thecomposition of the invention. Since many embodiments of the inventioncan be made without departing from the spirit and scope of theinvention, the invention resides in the claims hereinafter appended.

1. A method for decoding compressed data, comprising: receivingcompressed data; decoding the compressed data; detecting an error whiledecoding a first location in the compressed data; accessing a reentrydata set having a pointer to a second location in the compressed datafollowing the first location and decoding information that enablesdecoding to start from the second location; accessing the secondlocation in the compressed data; and using the decoding information inthe accessed reentry data set to continue decoding the compressed datafrom the second location.
 2. The method of claim 1, wherein the decodinginformation includes decoded data preceding output produced by decodingthe compressed data at the location addressed by the pointer andhistorical information used to enable decoding the compressed data atthe location addressed by the pointer.
 3. The method of claim 1, whereinthere are a plurality of reentry data sets, each having a pointer to onelocation in the compressed data and decoding information that enablesdecoding to start from the location addressed by the pointer, andwherein accessing the reentry data further comprises: determining one ofthe reentry data sets.
 4. The method of claim 3, wherein determining oneof the reentry data sets further comprises: determining the reentry dataset from the plurality of reentry data sets whose pointer addresses aclosest location to the first location in the compressed data followingthe first location.
 5. The method of claim 1, wherein the compresseddata is transmitted over a network from a transmitting system.
 6. Themethod of claim 5, further comprising: sending a request to thetransmitting system for a retransmission of compressed data includingthe first location after detecting the error; receiving a block of thecompressed data starting at a third location in the compressed data,wherein the pointer in one reentry data set addresses the thirdlocation, and wherein the block of the compressed data includes thefirst location; and using decoding information in the reentry data sethaving the pointer to the third location to decode the block of thecompressed data including the first location.
 7. The method of claim 6,wherein the block of the compressed data includes compressed data fromthe third location to the second location in the compressed data.
 8. Themethod of claim 6, wherein the compressed data is being decoded by thereceiver in parallel with receiving the data from the transmittingsystem.
 9. The method of claim 8, further comprising: receiving theblock of compressed data and the reentry data set having the pointer tothe third location in response to the request; and receiving reentrydata sets with compressed data after receiving the block of compresseddata, wherein before the error was detected, the reentry sets were nottransmitted with the compressed data.
 10. The method of claim 1, whereinthe compressed data is received from a storage medium.
 11. The method ofclaim 10, wherein the compressed data is archived in the storage medium.12. The method of claim 10, wherein the compressed data is received inresponse to a request from an application program or operating systemprocess executing on a computing device.
 13. A system for decodingcompressed data, comprising: means for receiving compressed data; meansfor decoding the compressed data; means for detecting an error whiledecoding a first location in the compressed data; means for accessing areentry data set having a pointer to a second location in the compresseddata following the first location and decoding information that enablesdecoding to start from the second location; means for accessing thesecond location in the compressed data; and means for using the decodinginformation in the accessed reentry data set to continue decoding thecompressed data from the second location.
 14. The system of claim 13,wherein the decoding information includes decoded data preceding outputproduced by decoding the compressed data at the location addressed bythe pointer and historical information used to enable decoding thecompressed data at the location addressed by the pointer.
 15. The systemof claim 13, wherein there are a plurality of reentry data sets, eachhaving a pointer to one location in the compressed data and decodinginformation that enables decoding to start from the location addressedby the pointer, and wherein the means for accessing the reentry datafurther performs: determining one of the reentry data sets.
 16. Thesystem of claim 13, wherein the compressed data is transmitted over anetwork from a transmitting system, further comprising: means forsending a request to the transmitting system for a retransmission ofcompressed data including the first location after detecting the error;means for receiving a block of the compressed data starting at a thirdlocation in the compressed data, wherein the pointer in one reentry dataset addresses the third location, and wherein the block of thecompressed data includes the first location; and means for usingdecoding information in the reentry data set having the pointer to thethird location to decode the block of the compressed data including thefirst location.
 17. The system of claim 16, wherein the compressed datais being decoded by the receiver in parallel with receiving the datafrom the transmitting system.
 18. An article of manufacture includingcode for decoding compressed data, wherein the code causes operations tobe performed, the operations comprising: receiving compressed data;decoding the compressed data; detecting an error while decoding a firstlocation in the compressed data; accessing a reentry data set having apointer to a second location in the compressed data following the firstlocation and decoding information that enables decoding to start fromthe second location; accessing the second location in the compresseddata; and using the decoding information in the accessed reentry dataset to continue decoding the compressed data from the second location.19. The article of manufacture of claim 18, wherein the decodinginformation includes decoded data preceding output produced by decodingthe compressed data at the location addressed by the pointer andhistorical information used to enable decoding the compressed data atthe location addressed by the pointer.
 20. The article of manufacture ofclaim 18, wherein there are a plurality of reentry data sets, eachhaving a pointer to one location in the compressed data and decodinginformation that enables decoding to start from the location addressedby the pointer, and wherein accessing the reentry data furthercomprises: determining one of the reentry data sets.
 21. The article ofmanufacture of claim 20, wherein determining one of the reentry datasets further comprises: determining the reentry data set from theplurality of reentry data sets whose pointer addresses a closestlocation to the first location in the compressed data following thefirst location.
 22. The article of manufacture of claim 18, wherein thecompressed data is transmitted over a network from a transmittingsystem, further comprising: sending a request to the transmitting systemfor a retransmission of compressed data including the first locationafter detecting the error; receiving a block of the compressed datastarting at a third location in the compressed data, wherein the pointerin one reentry data set addresses the third location, and wherein theblock of the compressed data includes the first location; and usingdecoding information in the reentry data set having the pointer to thethird location to decode the block of the compressed data including thefirst location.
 23. The article of manufacture of claim 22, wherein thecompressed data is being decoded by the receiver in parallel withreceiving the data from the transmitting system.